I haven't had any bad experiences yet but my parents got screwed around by one of our LDS after changing their mind on a purchase. Already participating in the $99 PADI OW courses, they each bought full sets of gear (BCDs, regulators, masks, fins, snorkels, boots, hoods, tanks, and wetsuits) and then decided they didn't want the tanks & regs (which were to be ordered and shipped to the LDS as they weren't in stock). My parents changed their minds before the tanks & regs were ordered by the LDS and after two weeks of nagging they finally got their money for the tanks & regs back. They also gave my dad a used BCD and told him it was new (there was minor wear on the suit, even some fading of the 'SHERWOOD' emblem on the front) and after another verbal bout with the shop owner, the BCD was finally replaced with a brand new one.
With 2 OW dives out of the way by this point the attitude of the LDS changed. All of a sudden they were being bumped from 'full classes' and not being called for open ones. After several weeks they inquired about hiring a private instructor to meet with the two of them so they could finish their last 2 dives. They were told 'no' by the shop, and only after asking for a referral to another local shop did they finally arrange for an instructor to meet them for dive #3. The instructor was an ass, constantly rolling his eyes at them and shaking his head. My dad actually scratched his dive because he was frustrated with the attitude he was getting from the instructor (my mom chose to go anyway and toughed it out). That was the last straw for them and eventually got a referral to a better outfit (after another few weeks of nagging).